1. Field of the Invention
The present invention is related to a spiral antenna having a wideband characteristic.
2. Description of the Related Art
Spiral antennas which radiate electromagnetic waves only in a forward direction of the antenna have spaces arranged between the antenna and cavity which correspond to frequencies being used. In this cavity-backed spiral antenna, the space between an antenna element and the cavity depends on a wavelength which corresponds to the used frequency. Therefore, the space becomes wider.
Given this factor, there has been suggested a microstrip spiral antenna which secures wideband characteristics by arranging a radio wave absorbent on the bottom of the cavity (refer to Jpn. Pat. Appln. KOKAI Publication No. 2000-252738). However, although the wideband characteristics can be secured by arranging the radio wave absorbent on the bottom of the cavity as in this antenna, no effect which reduces the height from the cavity to the antenna can be obtained. Therefore, there has been a problem that a high antenna mounting space became necessary.